Process for preparing multi-component nuclear fuels

ABSTRACT

A PROCESS FOR PREPARING URANIA-PUTONIA MICROSPHERES BY IMPREGNATING A MATRIX OF URANIA WITH A PLUTONIUM SALT SOLUTION. THE ABSORBED PLUTONIUM IS PRECIPITATED IN THE PORES OF THE URANIA MICROSPHERES WITH A SUITABLE PRECIPITANT. THE IMPREGNATED MICROSPHERES ARE THEN WASHED, DRIED AND SINTERED TO RECOVER THE MIXED OXIDE MICROSPHERES.

3,789,013 PROCESS FOR PREPARHJG MULTll-COMPONENT NUCLEAR FUELS LeonardV. Triggiani, Rockville, and Moises G. Sanchez, Severna Park, Md.,assignors to W. R. Grace & (10., New York, N.Y.

No Drawing. Continuation-impart of application Ser. No. 860,814, Sept.24, 1969, which is a continuation-in-part of application Ser. No.670,294, Sept. 25, 1967, now Patent No. 3,514,412. This application July26, 1971, Ser. No. 166,301 The portion of the term of the patentsubsequent to May 26, 1987, has been disclaimed Int. Cl. C09r 3/00 US.Cl. 252301.1 S Claims ABSTRACT OF THE DISCLOSURE A process for preparingurania-plutonia microspheres by impregnating a matrix of urania with aplutonium salt solution. The absorbed plutonium is precipitated in thepores of the urania microspheres with a suitable precipitant. Theimpregnated microspheres are then washed, dried and sintered to recoverthe mixed oxide microspheres.

CROSS REFERENCE TO OTHER APPLICATIONS This application is acontinuation-in-part of application Ser. No. 860,814 filed Sept. 24,1969, now US. Pat. 3,671,453 which in turn is a continuation in part ofapplication Ser. No. 670,294 filed Sept. 25, 1967, now US. Pat.3,514,412.

BACKGROUND OF THE INVENTION The preparation of fuel elements from solsby the microsphere route has resulted in products that have verydesirable physical properties. The microspheres can be sintered to veryhigh density at much lower temperatures than was possible when the fuelelements were prepared by conventional ceramic techniques. Themicrospheres range in size from a few microns up to 1000 microns or moreand provide a very convenient method of handling nuclear fuels.

There has been demand for fuel elements of binary and multi-componentstructure containing more than one material. Plutonia-urania fuels, forexample, are in demand as are the thoria and urania or plutonia fuelscontaining fissionable materials such as U-235, U-233 or Pu-239.

In the prior art processes these compositions have been prepared bycoprecipitation from solutions of salts of the respective metals or byphysical mixing of the dried oxide particle, followed by comminution,pressing, etc., by means of conventional ceramic techniques. They havealso been prepared from mixed oxide sols in a process wherein a solutionor sol of the second component is added to a sol of the first component.

The differences in the level of the radio-toxicity of.

the fertile matrix component and the fissionable second componentincreases the problems in preparing mixed oxide sol products by theseconventional techniques. Urania (U-238) can be handled safely withconventional laboratory equipment. Plutonium, on the other hand, canonly be handled in glove boxes and other sophisticated equipment. Thehandling problems are multiplied several fold when the mixed oxide fuelpreparation is scaled up for plant production.

BROAD STATEMENT OF THE INVENTION We have developed a process forpreparing these mixed fuel systems in which the two components arehandled separately in equipment designed to handle each 3,789,013.Patented Jan. 29, 1974 component. The system is a substantialimprovement over the conventional processing in which both thecomponents are processed together in each of the steps, so that the mostsophisticated equipment is necessary for each and for the entirequantity of material being proc essed.

In our novel process, we impregnated porous matrix particles with oxideprecursors of the fissionable component. This may be accomplished byimpregnating the spheres with sols or a solution of a salt of thedesired fissionable component followed by conversion to the oxide in thepores of the matrix. The particles can also be impregnated with theadditive in gaseous form (PuF UF etc.) followed by precipitation in thepores and conversion to the oxides.

The particles may be subjected to various degrees of drying. The spheresmay properly be termed gels in microspheroidal form. The term gel isthus applicable to describe our matrix materials.

Our process fills a long standing need for means of preparing a mixedoxide fuel wherein the fissionable oxide component is isolated to only afew process steps, so that the greatest amount of work can be done inconventional equipment. This system also avoids the problem ofcontamination of the equipment with the fissionable materials prior tothe final step of the process.

DESCRIPTION OF THE PREFERRED EMBODIMENT For purposes of simplicity ourprocess will be described as our preferred microsphere impregnationprocess. However, it is obvious that our process can be used forpreparing nuclear fuel particles in any desired shape or physical form.

In our preferred process, we prepare the fertile matrix microspheres(thoria or urania) and add the desired amount of the fissionablecomponent as a solution of a salt or as a sol of the component. Theimpregnated microspheres are then washed, dried and calcined to preparethe final product.

Broadly speaking, our preferred process comprises the following steps:

1. Selection and dissolution of the fuel raw materials.

2. Preparation of aquasols or suitably modified solutions of thesematerials.

, 3. Formation of microspheres from the sols or the suitably modifiedsolutions.

4. Addition of a fissionable additive into the microsphere product.

5. Washing, drying and sintering the microspheres containing theadditive.

In the first step of our process, the materials to be used as thefertile base material and the fissionable material are selected. Theprincipal matrix materials are natural, reprocessed or depleted urania(U238) alone, or in admixture with other materials such as thoria, forexample. Thoria (Th-232) may also be used as a fertile matrix material.The fissionable second component of the microspheres may be urania orplutonia, for example. Urania (U-238), for example, may be impregnatedwith plutonia (Pu-239) or the fissionable isotopes U- 233 or U-235. Amixed thoria-urania or plutonia fuel may be prepared by impregnatingthoria microspheres with U233, U-235 or Pu-239.

The fertile matrix material, urania or thoria, is first obtained as asolution of nitrate, chloride, etc. The solution is then converted tothe sol form. Suitable sols may be prepared by any of several methods.The preferred techniques for sol formation are:

1. Electrodialysis using anion permeable membranes.

2. Controlled hydrolysis with urea.

3. Ion exchange using resin in the hydroxide form.

4. Peptization of washed hydroxides with an acid.

5. Electrolysis of solutions, with oxidation of the anions to a volatilecomponent.

In the next step of our process, the sols are converted to microspheres.The method of preparing these microspheres is not part of thisinvention. It is covered in copending application Ser. No. 541,519,filed Apr. 11, 1966, now US. Pat. 3,331,785. Briefly, the processcomprises forming the sols into droplets and drying the droplets in acolumn of solvent passed in countercurrent direction to the solparticles. The formed microspheres are removed from the bottom of thecolumn and washed.

The matrix material may also be prepared as a powder or as microspheresor as larger sized spheroids in a process in which a solution of a saltof the matrix material is admixed with a water soluble resin thatincreases in viscosity in an alkaline medium. The droplets of solutionare fed into an aqueous alkaline solution to form microspheres orspheroids. The particles or spheroids are recovered, Washed and dried.

The microspheres or particles may also be prepared by any of the otherprocesses described in the technical and patent literature, providedthat the final product has suflicient porosity to retain the desiredamount of the fissionable component.

In our process, we believe that the thoria and urania microspheres whencontacted with the solution or sol containing the fissionable componentaccommodates the fissionable component solution within the voids in themicrosphere. The solution may then be converted to an insoluble form,dried and sintered.

The solution or sol of the fissionable material used to impregnate themicrospheres may be prepared in an inorganic or organic solvent. Anaqueous solution of the salt is preferred, since it is thereby possibleto achieve a higher concentration of fissionable solution. The preferredsalt is the nitrate; (PuVI, UVI), the chloride, sulfate, etc., can alsobe used in the preparation of the impregnation solution. Theimpregnation solution of fissionable material is prepared inconcentrations of about 0.1 to 700 grams per liter. The impregnation isetfected by contacting the microspheres with the solution for a periodof 1 to 300 minutes. When an organic solvent is used, the preferredsolvent is acetone. Other suitable solvents include diethyl ether,dibutyl ether, methylisobutylketone, tributylphosphate, trioctylamine,trilaurylamine, cyclohexyldilaurylamine, certain alcohols, etc.

When the fertile base microspheres are uranium dioxide, they may behyperstoichiometric in oxygen at this stage, due to the presence ofhexavalent uranium. Hexavalent uranium is more soluble in aqueous mediathen quadrivalent uranium. In that case, the microspheres must bereduced to the dioxide if the product is to be free of interparticlesludge, etc. This reduction can be carried out by using any suitabletechnique, such as hydrogen reduction, etc. However, this step can beomitted if a nonaqueous solvent is used to prepare the impregnant. Thehydrogen treatment is carried out at a temperature of 300 to 900 C. forabout 1 to about hours. In our process, this step may be completed atlow temperatures without destruction of the microsphere structure orporosity. The unsintered microspheres have pore volumes of from about0.01 to less than about 1 cc. per gram. The pore volumes of essentiallyall the microspheres are in the range of 0.1 to 0.99 cc. per gram.

In one acceptable technique the spheres are impregnated by slowly addingthe solution of the fissionable salt or the sol to the microsphereswhile they are being agitated. When the fertile matrix material isurania, a portion of the agitation is preferably provided by a flow ofargon or other inert gas that prevents the oxidation of urania to thehexavalent state. Any suitable inert gas, such as helium, argon, neon,nitrogen, etc., may be used. In the laboratory, it is convenient toagitate th m c ospheres by regulating the flow of gas in the areasurrounding the microspheres. Other mechanical techniques for agitationmay be used such as stirring, shaking, etc. Agitation is necessary,however, to permit homogeneous impregnation of the microspheres by thesolution or sol of the fissionable component. The amount of theimpregnant added to the microspheres is dependent on the finalconcentration of the fissionable material desired in the product and canbe conveniently controlled by adjusting the concentration of thesolution. In addition, we may use a second impregnation, if necessary,to introduce more impregnant.

When s oluti9ns are used, the microspheres are normally impregnated toincipient wetness. After impregnation, the microspheres are convenientlytreated with an organic solvent, such as hexanol, to remove water. It isobvious that other solvents besides hexanol can be used and the choicewill be dictated by the solvents used in preparing the solution of thefissionable component.

In yet another acceptable technique, the spheres may be impregnated bytotal immersion in the medium hearing the impregnant species. Such mediamay be either gases or liquids such as water, or organic solvents suchas acetone, carbon tetrachloride, tributylphosphate, etc.

After any excess water is removed from the microspheres, the fissionablecomponent is precipitated, conveniently as the hydroxide, in the voidsin the microspheres. For the purpose of our invention, the termhydroxide includes all fissionable metal hydrous oxides which areobtainable by the reaction of the fissionable metal cations withhydroxyl groups from basic solutions. Such metal hydrous oxides may bevisualized as the products obtainable by partial dehydration of thecorresponding hydroxide. In general, the dehydration is accompanied bypolymerization. A convenient formula to express the metal-hydrous-oxidefamily of compounds is metal (OH),; O where nt=valence of the metal m=degree of dehydration (O lnlt/Z) x=degree of polymerization (x21) and Oand H have their usual chemical symbolism. Obviously the fissionablecomponent can be precipitated as the carbonate, oxalate, etc. Thefissionable component may also be precipitated as the fluoride,phosphate, iodate or ferrocyanide.

An organic acid may be used as the precipitant. Examples of suitableorganic acid precipitants include benzoic, picrolinic, fumaric, sebasic,cinnamic, phenyl arsonic and salicylic.

The fissionable material may also be held in the voids of the particlesas insoluble complexes. Examples of suitable complexing agents include 8hydroxyquinioline, acetylacetonate and pyrogallol.

The precipitation is preferably eifected by contact with a nitrogencontaining basic compound such as ammonia or amines having less than 10carbon atoms in the molecule. A base that will not leave a contaminatingresidue on the impregnated product, such as gaseous or liquid ammonia,for example, is preferred. The precipitation is generally carried out byslurrying the microspheres in an aqueous ammonia solution. We preferconcentrated aqueous ammonia. However, concentrations of ammonia between5 and 30 weight percent may be used. Gaseous NH has also been used forthis purpose. Generally, the precipitation is complete in about 10minutes. However, shorter or longer times may be dictated by the type ofoperation and equipment being used.

The microspheres are then washed in deionized water to remove excessammonia, anions, hexanol or other solvent. Generally about 250 cc. ofdeionized water per gram microspheres is sufiicient to remove allimpurities. The microspheres are conveniently washed with hexanol and.

dried over a period of about 3 to hours while the temperature isincreased from room temperature to about 120 C.,The washing step may beomitted and the ammonia, Water, etc. removed by volatilization in thedrying step.

The sintering step is the final step of our process. Sintering ispreferably carried out in a hydrogen-nitrogen atmosphere by heating at300 to 700 C. for about 0.5 to 7 hours, followed by sintering foranother 0.5 to 6 hours at 1000 C. to 1800 C.

The matrix particles may, of course, be separated according to size byscreening or other techniques and only those particles falling in agiven size range impregnated with the fissionable materials.

Our invention is further illustrated by the following specific butnonlirniting examples:

EXAMPLE I Since fissionable materials involve many handling difficultiesand are not available for general laboratory investigation, uraniamicrospheres were impregnated with ceria and thoria. Thoria was chosenas an analogue of plutonia because it has a similar crystal structure.Ceria was chosen as a good analogue for the wet chemistry of plutonia.

In this example, the urania microspheres were impregnated with a thoriumnitrate solution. The urania microspheres were prepared according to theprocess described in U.S. Ser. No. 541,519, now US. Pat. 3,331,785. Themicrospheres were pretreated by heating to 500 C. in a hydrogenatmosphere for a period of 3 hours. This treat-. ment converted anyhexavalent urania to quadrivalent urania.

The impregnation solution was made up by dissolving grams of thoriumnitrate in 15 ml. of water. This solution was added dropwise to the U0microspheres. The impregnation was carried out in'a dry box in an argonatmosphere. The microspheres were stirred as the impregnation solutionwas added. A total of about 12.9 cc. of solution wasrequired to bringthe 202 gram sample of urania microspheres to incipient wetness. Thesample was then ammoniated by contacting with 2.5 cc. of 1:5 ammoniumhydroxide. The ammonia was removed very gently by decantation. Noprecipitate was observed in the ammonia filtrate. The spheres at thispoint were whole and entire with no precipitate on the outside surfaces.To all external appearances these spheres appeared the same as beforeimpregnation.

The impregnated spheres were dried under an infrared lamp for 4 hours.Following this drying, the sample was vacuum dried using the followingschedule:

/2 hr. at 40 C.

1 hr. at 60 C.

1 hrs. at 80 C. 1 /2 hrs. at 100 C. 1 hrs at 120 C.

The microspheres were then cooled in a vacuum at room temperature overan 8 hour period at which time the vacuum was broken using nitrogen. Thespheres appeared intact. The dried microspheres were than sintered usingthe following cycle. The temperature was increased to 350 C. in one hourand from 350 C. to 500 C. in two hours and held at 500 C. for one hour.The temperature Was then raised to 1400 over a 1 /2 hour period. Thetemperature Was held at 1400 C. for 1 hours and quenched. The entirecycle was completed in a hydrogen atmosphere.

There was no evidence of inter-particle sludge accumulation and no smallparticulate crystals appeared on the surface of the microspheres. Thesample was analyzed for thorium by an oxalate precipitation method andfound to contain 10.51 weight percent thorium. The samples were alsoanalyzed by X-ray diffraction and found to have a single phase pattern.The X-ray diffraction lines were very sharp indicating completeness ofsolid solution. The unit cell constant (a was calculated and found to be5.4851 A. The weight percent solid solution aproximated from this unitcell corresponds to 13 weight percent ThO A metallographic mount wasprepared of the sample. Examination showed the microspheres to beuniform, finegrained material in cross section.

An additional sample of the microspheres was mounted in a special resin,ground to expose the cross section and final-polished. This sample wassubjected to electron microprobe analysis. The controls of themicroprobe instrument were set to correspond to their highestsensitivity for thorium. Tracings were made across several microspherecross sections. The distribution of thorium was found to be homogeneous,uniform from sphere to sphere, and within each sphere.

The density of the microspheres was measured by mercury displacement andfound to be 10.64 g./cc.

EXAMPLE II This example describes a process for preparing ceriaimpregnated urania microspheres.

The urania microspheres used for the impregnation were prepared in thesame manner as the microspheres described in Example I. They werepretreated in a hydrogen atmosphere to reduce the urania tostoichiometric U0 A solution was prepared to contain 20 grams of ceriumnitrate in 15 ml. of water. The microspheres were then impregnated toincipient wetness with the cerium nitrate solution. The impregnation wascarried out using the same-equipment and technique as in Example I. Thesample was ammoniated by contacting with about 3.5 cc. of concentratedammonium hydroxide diluted with water in the ratio of l to 5. Theammonia was removed by decantation. No precipitate was observed in theammonia filtrate. The microspheres were dried under an infrared lamp for5 hours and then washed for 8 hours. After washing, the sample wasvacuum dried as in Example I.

The dried microspheres were then sintered using the following cycle. Thetemperature was increased to 1000 C. slowly in hydrogen atmosphere overa period of 4% hours. The temperature was increased to 1400 C. over aperiod of one hour and held at 1400 to 1485 C. for 1%-hours. The productwas then quenched under hydrogen. There was no evidence ofinter-particle sludge accumulation and no small particulate crystalsappeared on the surface of the microspheres. The density of the productwas 10.7 g./ cc. A sample was submitted for cerium analysis and found tocontain 1.9 w/o CeO A metallographic mount was prepared as in Example I.Examination showed the microspheres to be uniform, fine-grained materialin cross section. The microsphere samples were then subjected to X-raydiffraction analysis. The unit cell calculation indicated complete solidsolution. The X-ray line widths indicated complete solid solution. TheX-ray line widths indicated single phase material. A sample ofmicrospheres was prepared for electron microprobe analysis as in ExampleI. The cerium content was found to be homogeneous and uniform fromsphere to sphere and within each sphere.

EXAMPLE III This example illustrates the preparation of a thioriaimpregnated U0 sphere using an improved washing technique.

The process was exactly the same as described in Examples I and II. Inthis case, a sample of urania was impregnated with a water solutionprepared by dissolving 20 grams of thorium nitrate in 15 ml. of water.The microspheres were impregnated to incipient wetness. The microsphereswere then washed by immersion in dry hexano'l to remove the water.Hexanol was removed by very gentle vacuum filtration so as not to damagethe impregnated microspheres. The sample was ammoniated by contactingthe microspheres with concentrated ammonium hydroxide. The sample wasthen de-watered and washed with water for 8 hours. The washed sample wasthen vacuum dried as described in Examples I and II. The sample wassintered in hydrogen according to the following cycle. The temperaturewas slowly raised to 1000 C. over a period of 30 minutes and held at1400 to 1485 C. for 2 hours. The product was then cooled under hydrogen.

The product was submitted for thorium analysis and found to contain 4.52w/o thorium. The product had a density of 10.73.

EXAMPLE IV After having worked out the technique for impregnatingmicrospheres with a fissionable material using the thoria and ceriaanalogues, a urania microsphere was 1mpregnated with plutonia.

A representative group of pure porous U microspheres (1 gram) was placedin a fritted disc filtration funnel in a glove box. This U0 microspheresample was unsintered microsphere prepared according to processdescribed in U.S. Ser. No. 514,519. The microspheres were pretreated byheating in a hydrogen atmosphere at 500 C. for about 3 hours. A stockplutonium solution about 6 molar in nitric acid was prepared containingapproximately 60 grams plutonium per liter. A 2.2 milliliter portion ofthis stock solution was diluted to 4.4 ml. with deionized water toprepare the impregnation solution. This solution was added dropwise tothe U0 microspheres. The addition was carried out in a glove box in anair atmosphere. The microspheres were constantly agitated during theaddition by a flow of argon through the funnel. A total of about 0.40cc. of solution was required to bring the spheres to incipient wetness.

The spheres were then washed with approximately 30 cc. of dry hexanol toremove the water. The hexanol was removed by vacuum filtration. Thisprocess was accomplished very gently so as to prevent harm to theimpregnated microspheres.

The sample was then ammoniated by contacting with about 90 cc.concentrated aqueous ammonia. The contact time was 15 minutes. Theammonia was then removed very gently by vacuum filtration. Noprecipitate was observed in the ammonia filtrate. The spheres at thispoint were whole with no precipitate on the surfaces. The impregnatedmicrospheres were then washed with 500 ml. deionized water over a 4 hourperiod during which time no physical change in the microspheres wereobserved.

Following washing, the sample was vacuum dried using the followingschedule:

/2 hr. at 40 C.

1 hr. at 60 C. 1 /2 hr. at 80 C. 1 /2 hr. at 120 C.

The microspheres were then cooled under vacuum to room temperature overan 8 hour period at which time the vacuum was broken using nitrogen. Thespheres still appeared intact to the naked eye.

The microsphere batch was then transferred to a molybdenum tray forfinal sintering. The sintering atmos phere was a 50-50 volume mixture ofhydrogen and nitrogen. The sintering process utilized a horizontal zonefurnace which was set up as follows: a 600 C. zone 2 feet long, atransition zone of about 2 /2 feet, and a high temperature zone at 3,050F. of about 4 feet. The tray was passed through the furnace at a rate ofnine inches per hour. The molybdenum tray containing the microspherestraveled through the furnace over a twenty-four hour period. The sphereswere at 3,050" F. for a total of about 5 to 6 hours. Microspheres wererecovered having good sphericity. There was no evidence ofinter-particlesludge accumulation, and no small particulate crystalsappeared on the surface of the microspheres.

The density of the product was determined using a xylene pycnometer andfound to be 10.43 g./cc. The microspheres were analyzed for plutoniumcontent. This determination was made by dissolving the microspheres in amixture of nitric and hydrochloric acid and adsorbing the solution in anion exchange resin column. The plutonium was eluted from the column anddetermined by titration. The microspheres were found to contain 1.2weight percent plutonium calculated as metal or 1.36 weight percentcalculated as PuO EXAMPLE V This example illustrates the impregnation ofurania microspheres by an immersion technique wherein the microsphereswere immersed in a solution of thorium tetrachloride.

A solution of thorium tetrachloride was prepared to contain 9.53 Weightpercent thoria (ThO The density of the solution was 1.133. A series ofruns were completed in which urania microspheres, prepared according tothe general process described in U.S. Pat. 3,331,785, were placed in anevaporating dish containing the thoria solution. In runs I and II, 2grams of the spheres were immersed in 1 cc. of the thoria solution. Inruns III and IV, 1 cc. of the thoria solution was diluted with an equalamount of water and 4 grams of urania spheres were then immersed in thediluted solution. In runs V and VI, 1 cc. of the solution was dilutedwith an equal volume of water and the solution and 2 grams of the U0spheres were immersed therein. The spheres were analyzed for the weightpercent thoria. The data collected in this series of runs is It isapparent from a review of these data that a substantial percentage ofthorium can be incorporated into microspheres by the immersiontechnique.

EXAMPLE VI A series of experiments were completed in which U0microspheres, prepared by the general technique described U.S. Pat.3,331,785, were impregnated with thorium nitrate solution.

A solution of thorium nitrate was made up by dissolving 60 grams of thesalt in 300 ml. of water. The resulting solution had a density of 1.344and contained 9.6 weight percent thoria. The urania microspheres used inthis run were pretreated by heating in a hydrogen atmosphere using thetechnique described in Example V. Six runs runs were completed toinvestigate the effect of time and concentration on the amount ofimpregnant taken up in the microspheres. In each of these runs, one gramof urania microspheres were exposed to 5 cc. volumes of thorium nitratesolution for a period of 2 to 300 minutes. After exposure for the timenoted, the thorium nitrate solution was removed by suction filtration.The spheres were then washed in hexanol for 5 minutes to remove theexcess water from the surface and the hexanol was removed by suctionfiltration, then ammoniated by immersion in concentrated ammoniumhydroxide for 15 to 20 minutes. The excess ammonia solution was removedby suction filtration. The samples were then washed with water for 8hours and the water removed by suction filtration. The samples weredried for 7 /2 hours under vacuum and sintered at 1400 C. for 2 hoursunder hydrogen atmosphere.

The thoria content of the microspheres was determined by dissolving themicrospheres in acid and separating the thoria by oxalic acidprecipitation, followed by analysis of the oxalate precipitate todetermine the amount of thoria present in each of the spheres. The datacollected in this series of runs is set out in the table below:

1 2.5 cc. solution, 2.5 cc. H2O.

It is obvious from a review of these data that the time of contact andthe volume of the thorium solution are very important. The microspheresthat were contacted with 5 ml. of the solution of a period of 2 minutescontained no thoria. Increasing the time to 60 and 300 minutes resultedin substantial impregnation with the nitrate solution. Diluting thesolution and using 2.5 ml. solution for a period of 2 minutes did notresult in any appreciable adsorption of the thon'a nitrate solution intothe pores of microspheres.

EXAMPLE VII This example describes a typical sintering cycle for thoriaimpregnated urania microspheres where the impregnation was carried outby the immersion technique. Four grams of urania microspheres, preparedby the general technique described in US. Pat. 3,331,785, were placed inan evaporating dish and contacted with 2 grams of thorium chloridesolution, prepared to contain 9.53 weight percent thoria. Themicrospheres were in contact with the solution for about 4 to 6 hours.The spheres were removed and sufiicient quantity of a 1 to 3 ammom'umhydroxide solution was added to cover the microspheres. The spheres wereallowed to stand in the ammonia solution for a period of about 16 hours.At the end of this period, the microspheres were removed, washed withwater and dried for a period of 7 /2 hours in a vacuum oven operated ata temperature of 120 C. The microspheres were then placed in a furnaceand sintered in an atmosphere of hydrogen. The following sintering cyclewas used:

The furnace was raised from room temperature to 500 C. over a period of1 hours, maintained at 500 C. for 3 hours, raised to 1400 C. over aperiod of 1 hour and maintained at 1400 C. for 2 hours. The sample wasquanched, cooled in hydrogen and submitted for analysis. Themicrospheres were found to contain 4.9 weight percent thorium by theX-ray spectroscopy technique.

EXAMPLE VIII When plutonium is precipitated in the pores of a gelsubstrate, the skeletal thickness may be thought of as a fertile matrixbarrier separating the plutonium precipitated into the pores. Onsintering, the plutonium will diffuse through the skeletal wall. We maytherefore conclude that, on an average, about one half of the skeletalwall thickness will represent the maximum diffusion path necessary forthe plutonium (or other fissionable component) and the fertile matrixmaterial to achieve homogeneous solid solution.

It is quite clear that gel materials having thin skeletons are desirableas impregnation substrates. A series of calculations were made to definethese properties of our matrices.

The surface area and pore volumes of a series of thoria sol residuesprepared by the electrodialysis technique were measured. In this series,the surface areas varied from 85 square meters per gram to 127 squaremeters per gram; the pore volumes from 0.11 to 0.13 cubic centimetersper gram.

The pore volume is equal to the void space per gram. The specific volumeis then equal to:

10 Pore volume plus skeletal volume, or in this case (Where the porevolume equals 0.13) to 0.23.

The porosity can be calculated using the formula:

Pore volume (co/per gram) Total volume (ca/per gram) Using the datacollected from sols prepared by electrodialysis, we find that, in thiscase, our microspheres have porosities of: 41 to 57%.

The skeletal thickness (6) can be calculated, the formulae varydepending on the model used. The cylinder is an often accepted model.The volume (V) of a cylinder can be calculated from the formula:

Porosity The skeletal thickness (9) is thus:

Volume 2 Surface Area Using the surface area and pore volume datareferred to above we found our matrix materials have a skeletalthickness (6) of from 7.9 A. to 118 A.

A basic characteristic of our materials is a combination of surface areaand pore volume which will minimize the diffusion path during thesintering steps. In general, diffusion paths of less than 500 A. arerequired for homogeneous solid solutioin formation. Of course, theshorter the diffusion path, the easier the attainment of solid solution.In our process we prefer to use materials with diffusion paths of lessthan 500 A., preferably less than 100 A.

Our fertile base materials can thus be characterized as having:

Porosities of 10% to An average diffusion path of less than 500 A.,preferably less than A.

What is claimed is:

1. A process for preparing a mixed urania-plutonia nuclear fuel whichcomprises the steps of:

(a) contacting unsintered urania microspheres having pore volumes ofabout 001 up to about 1.0 cc. per gram with a plutonium salt solutionfor a period of time suflicient to insure absorption of the plutoniuminto the interstices in the urania microsphere,

(b) converting the absorbed plutonium salt to the hydroxide, thecarbonate or the oxalate, and

(c) drying, sintering and recovering the fuel as oxide microspheres.

2. A process for preparing a mixed urania-plutonia nuclear fuel whichcomprises the steps of (a) contacting unsintered urania microsphereshaving pore volumes of about 0.1 up to about 0.99 cc. per gram with aplutonium salt solution for a period of time sufficient to insureabsorption of the plutonium into the interstices in the uraniamicrosphere,

(h) converting the absorbed plutonium salt to the hydroxide, thecarbonate or the oxalate, and

(c) drying, sintering and recovering the fuel as oxide microspheres.

3. The process according to claim 1 wherein the microspheres arecontacted with an aqueous solution of plutoniurn nitrate, chloride orsulfate in a concentration of 0.1 to 700 grams per liter for a period of1 to 300 minutes.

4. The process according to claim 1 wherein the urania microspheres arecontacted with a plutonium salt solution in an organic solvent for aperiod of 1 to 300 minutes.

5. The process according to claim 1 wherein the organic solvent isselected from the group consisting of acetone, diethyl ether, dibutylether, methyl isobutylketone, tributylphosphate, trioctylamine,trilaurylamine and cyclohexyldilauryla'mine, and the plutonium ispresent in an amount equal to 0.1 to 700 grams per liter.

6. The process according to claim 1 wherein the plutonium adsorbed inthe microspheres is converted to the hydroxide by contact with anitrogen containing basic compound, selected from the group consistingof ammonia or amines having less than 10 carbon atoms in the molecule.

7. The process according to claim 1 wherein the plutonium is convertedto the hydroxide by contact with a 5 to 30 weight percent ammoniumhydroxide solution.

8. The process according to claim 1 wherein the plutonia containingmicrospheres are washed with an alcohol, dried and sintered.

9. The process according to claim l'wherein the microspheres are washedwith hexanol and dried over a period of about 7 hours, While thetemperature is increased from room temperature to 120 C.

10. The process according to claim 1 wherein the dried microspheres aresintered in a reducing atmosphere for a period of about 4 to 6 hours ina cycle where the temperature was increased from 350 C. to 1400 C.

11. The process according to claim 1 wherein the dried microspheres aresintered while being moved through a furnace heated to a temperature offrom about 600 F. to 3000 F. over a period of about 24 hours.

12. A process for preparing a mixed thoria-plutonia nuclear fuel whichcomprises contacting unsintered thoria microspheres having a pore volumeof about 0.01 up to about 0.99 cc./g. with a plutonium salt solution fora period of time sufficient to insure absorption of the plutoniumsolution into the interstices in the thoria microspheres, converting theabsorbed plutonium salt to a compound selected from the group consistingof the hydroxide, carbonate or oxalate, drying, sintering and recoveringthe fuel as oxide microspheres.

13. A process for preparing a mixed thoria-urania nuclear fuel whichcomprises contacting unsintered thoria microspheres having a pore volumeof about 0.01 to 0.99 gram per cubic centimeter with a uranium oruranium salt solution for a period of time sufiicient to insureabsorption of the uranium solution into the interstices in the thoriamicrospheres, converting the absorbed uranium salt to a compoundselected from the group consisting of the hydroxide, carbonate oroxalate, drying, sintering, and recovering the fuel as oxidemicrospheres.

14. The process according to claim 13 where the urania is U O 15. Theprocess according to claim 13 wherein the urania is U O References CitedUNITED STATES PATENTS 3,320,177 5/1967 Halva 25230l.1 3,320,178 5/1967Dewell 252-301.1

CARL D. QUARFORTH, Primary Examiner R. L. TATE, Assistant Examiner U.S.Cl. X.R. l7689; 2640.5

